Numerous applications for phase change energy storage materials exist where excess thermal energy is lost due to an inability to store it, or where removal of this heat is desired. Examples include the regulation of food temperatures during transport or storage to prevent spoilage or maintain a desired temperature. Phase change materials (PCMs) can be used to store and release thermal energies through phase transformations—most commonly melting and crystallization. The energy which is stored and released in these transitions is known as latent heat, and latent heat storage is among the most effective techniques for storing and/or regulating thermal energy. By selecting a PCM possessing a phase change at the desired temperature and with the required enthalpy of phase change, thermal fluctuations can be regulated.
Different temperature ranges between the classes of PCMs generally dictates which type of PCM will be suitable for a specific application. PCMs are most often divided into classifications based on their chemical composition. Inorganic PCMs such as hydrated and molten salts are used for high temperature applications, while organic PCMs such as fatty acids and paraffin waxes are used at lower temperatures. Paraffin waxes, which consist of a series of linear and branched saturated hydrocarbon molecules which are solid at room temperature, are one of the most widely used PCMs. Paraffin waxes are formulated based on different boiling fractions from petrochemical refineries, and a single fraction generally contains multiple isomers and alkane chain lengths. The melting point of a paraffin wax is dependent on the number of carbon atoms in the paraffin, and it generally increases with the average carbon number.
Due to the environmental implications and increasing economic costs associated with limited oil reserves and corresponding increasing costs associated with petrochemicals, renewable alternatives for paraffin wax are attracting significant attention. Analysis of the constituents of paraffin waxes in candles shows that such materials often contain hazardous components, such as toluene, trichloroethylene and/or a variety of alkanes and alkenes, many of which have unknown safety implications. Due to these toxicity issues and the non-renewability of the feedstock, interest in renewable PCMs from fatty acids and fatty acid esters has been growing. Fatty acids are suitable candidates for the substitution of paraffin wax in many applications—they may be non-toxic, are renewable. The fatty acid derived PCMs produced to date typically have melting temperature and latent heat values lower than those of paraffin wax, resulting in lower energy storage densities. Blends of fatty esters with varying melting points and latent heat values between 110-140 J/g has been reported. Although the PCMs that were produced are renewable, the melting points and latent heat values compare unfavorably with those of paraffins.